We have described how rat mammary cancer MTW9 can be grown as two lines depending on host endocrine environment. One line alwasy regresses after ovariectomy, the other does not. Both tumor lines posses estradiol receptor but the receptors have been shown to be different by 2 chemical techniques; binding of receptor to DNA and the rate of ether-catalyzed dissociation of hormonae receptor complex. We now wish to apply these techniques to human material. To this end we have devised a double-isotope technique for comparing 3H and 125I labeled estradiol tumor receptors. Changes in isotope ratio on binding and elution to DNA, or rates of dissociation of estradiol from ER should allow detailed study of biopsy material. Aside from possible improvement in predicting the effects of hormonal treatment, these data may allow improved classification of human breast cancer predictability of response to chemotherapy.